當使用很多路徑和繪製對象時，Android畫布會被清理

Question

有人能解釋我爲什麼地球上的這段代碼不畫每個對象嗎？

public class A extends View { 
private Paint paint = new Paint(); 
private Path path = new Path(); 
ArrayList<Pair<Path, Paint>> paths = new ArrayList<Pair<Path, Paint>>(); 


public A(Context context) { 
    super(context); 
} 

@Override 
protected void onDraw(Canvas canvas) { 
    for (Pair<Path, Paint> p : paths) { 
     canvas.drawPath(p.first, p.second); 
    } 
    canvas.drawPath(path, paint); 
} 



@Override 
public boolean onTouchEvent(MotionEvent event) { 
    float eventX = event.getX(); 
    float eventY = event.getY(); 
    paint.setAntiAlias(true); 
    paint.setStyle(Paint.Style.STROKE); 
    paint.setStrokeJoin(Paint.Join.ROUND); 
    paint.setStrokeWidth(3f); 

    switch (event.getAction()) { 
     case MotionEvent.ACTION_DOWN: 
      int color = Color.rgb(new Random().nextInt(255), 
           new Random().nextInt(255), 
           new Random().nextInt(255)); 
      paint.setColor(color); 
      path.reset(); //new stroke, get old one erased 
      int historySize = event.getHistorySize(); 
      for (int i = 0; i > historySize; i++) { 
       path.moveTo(eventX, eventY); 
      } 
      path.moveTo(eventX, eventY); 

      return true; 
     case MotionEvent.ACTION_MOVE: 
      path.lineTo(eventX, eventY); 
      return true; 
     case MotionEvent.ACTION_UP: 
      path.lineTo(eventX, eventY); 
      // End of stroke, add this to the collection 
      paths.add(new Pair<Path, Paint>(path, paint)); 
      break; 
     default: 
      break; 
    } 
    // Schedules a repaint. 
    invalidate(); 
    return true; 
} 

}

我趕上與每次的onTouchEvent招我創建存儲在一對一個不同的路徑/油漆ojects。可悲的是，在我的OnDraw中，當我嘗試繪製它們時，它全部失敗。 我讀過一些主題，但沒有找到正確的答案。每次有人建議創建和工作在位圖中並將其繪製到屏幕上，但我想避免此解決方案。

感謝您的幫助！

Answer 1

的問題是，你總是使用相同的Path和Paint對象。您應該每次創建新的Path和PaintMotionEvent.ACTION_DOWN被觸發

Answer 2

刪除切換條件中的path.reset。繪畫作品。但即使是之前的平局，顏色也會改變。您重置正在清除之前繪製的路徑。

我會每次使用不同的對（路徑和繪畫）來繪製。

如果你想使用畫布繪製下面的代碼工作就好了。 我也會添加一個顏色選擇器，以允許用戶選擇他選擇的顏色。

public class MyView extends View { 

    private static final float MINP = 0.25f; 
    private static final float MAXP = 0.75f; 

    private Bitmap mBitmap; 
    private Canvas mCanvas; 
    private Path mPath; 
    private Paint mBitmapPaint; 

    public MyView(Context c) { 
     super(c); 

     mPath = new Path(); 
     mBitmapPaint = new Paint(Paint.DITHER_FLAG); 
    } 

    @Override 
    protected void onSizeChanged(int w, int h, int oldw, int oldh) { 
     super.onSizeChanged(w, h, oldw, oldh); 
     mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888); 
     mCanvas = new Canvas(mBitmap); 
    } 

    @Override 
    protected void onDraw(Canvas canvas) { 
     canvas.drawColor(0xFFAAAAAA); 

     canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint); 

     canvas.drawPath(mPath, mPaint); 
    } 

    private float mX, mY; 
    private static final float TOUCH_TOLERANCE = 4; 

    private void touch_start(float x, float y) { 
     mPath.reset(); 
     mPath.moveTo(x, y); 
     mX = x; 
     mY = y; 
    } 
    private void touch_move(float x, float y) { 
     float dx = Math.abs(x - mX); 
     float dy = Math.abs(y - mY); 
     if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) { 
      mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2); 
      mX = x; 
      mY = y; 
     } 
    } 
    private void touch_up() { 
     mPath.lineTo(mX, mY); 
     // commit the path to our offscreen 
     mCanvas.drawPath(mPath, mPaint); 
     mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SCREEN)); 
     // kill this so we don't double draw 
     mPath.reset(); 
    } 

    @Override 
    public boolean onTouchEvent(MotionEvent event) { 
     float x = event.getX(); 
     float y = event.getY(); 

     switch (event.getAction()) { 
      case MotionEvent.ACTION_DOWN: 
       touch_start(x, y); 
       invalidate(); 
       break; 
      case MotionEvent.ACTION_MOVE: 
       touch_move(x, y); 
       invalidate(); 
       break; 
      case MotionEvent.ACTION_UP: 
       touch_up(); 
       invalidate(); 
       break; 
     } 
     return true; 
    } 
} 

Answer 3

您只需要使用一個Path和Paint對象。

在這整個事情的詳細教程，其實也可以在這裏找到： Making a basic single touch drawing app on Android - Creative Punch